The present invention pertains to the radio communication art and, more particularly, to a means for limiting the peak modulation of an amplitude modulated signal.
Systems for amplitude modulating a radio frequency carrier are well known in the communication art. A common system employs a modulating transformer having primary and secondary windings. The primary winding is driven from the modulating signal, ordinarily an audio information signal, with the secondary feeding to a radio frequency modulating stage. The modulating stage amplitude modulates a generated radio frequency carrier by the transformed information signal.
Prior art amplitude modulating systems employing a modulating transformer have suffered from several problems. Firstly, amplitude modulating schemes are normally limited to a maximum index of modulation, commonly 100 percent. Modulation above this level may be legally restricted, or may result in undesirable distortion in the transmitted signal. However, transmitted power is directly related to the level of the index of modulation, whereby for maximum radiating area a transmitter should operate at a high percent modulation. Thus, conventional modulating transformer schemes resulted in a tradeoff of either over modulating the carrier on information signal peaks, thereby producing adjacent channel splatter and signal distortion, or under modulating the RF carrier with the result that the maximum radiating range of the transmitter is significantly reduced.